That's certainly a more concise way of writing that. Of course you're right that you could just decide that "instance" was your constructor instead of "new." But "new" is the usual, conventional way of naming constructors both in general Perl and in Moose-based classes specifically, and I think it's reasonable to want to maintain that convention much of the time. I don't think there's anything wrong with the factory approach but I don't think it's the only one that makes sense.
In any event, I think the cached nature of the object is actually an implementation detail that's internal to the class, for many classes with immutable objects. Sometimes caching is going to be worth the tradeoff in memory and lookup time, and sometimes it won't, and I shouldn't need to know whether some class or other caches its objects as long as the ultimate behavior is the same, and I don't think one should use different constructors based on that. In old-style Perl OO, this would not be an issue.